Current transformers generally operate with a low-resistance burden of less than one ohm, for example when using an ammeter in the secondary circuit. Since the secondary voltage is proportional to the burden, the voltage at the secondary terminals might assume unacceptably high values in case of an idle secondary connection. In order to achieve protection of the transformer in the event of an interruption of its secondary side short-circuit, for example when the metering device including its shunt resistor is removed, circuit breakers have been known for re-establishing the short-circuit upon occurrence of unacceptably high secondary voltages.
Voltage peaks that may occur in case of an open secondary circuit of the current transformer might be very high and therefore life-threatening and may moreover result in damage of the current transformer.
As a remedy, short-circuiting devices have been known which are short-circuiting instantaneously, for example using diodes or relays, or with a delay. For delayed short-circuiting, implementations are known which periodically short-circuit, for example using a thermistor, or which permanently short-circuit, for example upon softening of a spacer.
Devices that are short-circuiting when a secondary cover is lifted or a connector is unplugged are problematic as they do not have any effect in case of an interruption elsewhere in the secondary circuit.
Relays can be designed so as to automatically re-enable the current transformer secondary circuit once the interruption has been rectified, and that without major delays.
However, usually such a device cannot be accommodated in a secondary terminal box of a current transformer. Moreover it is expensive and does not always meet the high operational requirements in terms of operational safety and weather resistance.
From FR 1 178 783, for example, a circuit breaker is known wherein in the event of unacceptably high secondary voltages a voltage-dependent circuit element actuates a switch powered by the current transformer. The secondary winding and the burden of the converter have connected in parallel thereto a series connection of two antiparallel-connected rectifiers and a relay implemented as a thermal overload relay. If the secondary voltage reaches a critical value the relay is responsive to close a contact which establishes a short-circuit path bridging either only the rectifiers, or the series connection of rectifiers and relay.
A drawback of this known circuit breaker is its high inertness so that both the response and the restoration of the normal operating condition after the decay of the excessively high secondary voltage are effected with a time delay. Furthermore, due to its size it is difficult for such a circuit breaker to be accommodated in the secondary terminal box of the current transformer, moreover it is expensive and problematic in terms of operational safety.